1. Field of the Invention
The invention is in the field of Nuclear Magnetic Resonance (xe2x80x9cNMRxe2x80x9d) tools. More specifically, the invention pertains to new designs in NMR measuring devices in the application of oil well logging.
2. Description of the Related Art
Nuclear Magnetic Resonance has uses in many areas, including the fields of medicine, non-destructive testing, and in well logging in the oil exploration industry. In the well logging industry, NMR is used in determining properties such as porosity of the material, permeability, the bound liquid volume, the clay bound volume (CBW) and bulk volume irreducible (BVI), as well as formation type and oil content.
A simple NMR device used in well logging uses a permanent magnet to apply a static magnetic field to a desired volume of investigation. Many magnet arrangements and field geometries have been proposed in prior art. In U.S. Pat. No. 4,350,955 to Jackson et al., two cylindrical magnets are placed end to end with north poles facing each other and are separated by a gap. This configuration produces a field in the center of this gap which is extending substantially radially outward. There are other designs which include spatial arrangements of multiple magnets. U.S. Pat. No. 4,717,868 to Taicher et al. and U.S. Pat. No. 4,710,713 to Shtrikman et al. show side-by-side arrangements of multiple magnets in order to design regions of magnetic fields where the field lines are substantially perpendicular to the longitudinal direction of the device.
The principle of NMR works because atomic nuclei contain magnetic moments associated with their nuclear spin. In the absence of an applied magnetic field, thermal fluctuations cause these moments to have random orientations in space. When these nuclei are subjected to a static magnetic field, the magnetic moments tend to align either parallel or anti-parallel to this applied field.
The permanent magnet of the NMR tool establishes the direction of orientation of the magnetic moments in a region being investigated. Typically in the art, a transmitter coil is placed in this region in order to induce a RF magnetic flux into this region by means of the circuitry to which it is attached. The transmitter coil is oriented such that the magnetic field it induces into the volume lies substantially in the plane that is perpendicular to the static magnetic field. A receiver coil is also placed in this region. In prior art, the transmitter coil and the receiver coil are the same. If the transmitter coil is separate from the receiver coil, the magnetic field produced by the coils must still be substantially perpendicular to the static field, but the coils need not share the same orientation. By applying a RF magnetic field perpendicular to the direction of the static field, we can xe2x80x9cflipxe2x80x9d the nuclear spin vectors out of their alignment with the static field.
Typically in the art, the transmitter coil induces a RF magnetic pulse whose duration is timed to reorient the magnetic moments of the nuclei along a direction that is perpendicular to both the direction of the static field of the permanent magnet and to the direction of the applied RF pulse. Once the spin moments are perpendicular to the static field and the RF pulse is removed, the moments undergo two notable processes. Firstly, the spins will realign along the direction of the static magnetic field. This decay back along the direction of the static field occurs over a characteristic time scale called the spin-lattice relaxation rate, T1. Secondly, since the magnetic moments are non-aligned with the static field, they experience a perpendicular force which causes them to precess around the static field. The rate of precession is known as the Larmor frequency and is proportional to the strength of the static field.
Immediately following the application of the xe2x80x9cflippingxe2x80x9d RF magnetic field, the spin vectors are all pointing in the same direction, and ideally as they precess, they should continue to point in a common direction. In real situations, the strength of the static field is inhomogeneous in space. As a result, the spins will tend to precess at different rates. The different precession rates cause the vector sum of the magnetization in the plane of the spins to decay to zero. This decay of the spin magnetization in the plane perpendicular to the static field is known as the free induction decay (FID) and is characterized by its decay rate, T2*. A simple method comprised of another magnetic pulse with twice the duration of the first pulse flips the spin vectors 180 degrees. After the flip, the leading spins now find themselves behind the other spins and the lagging spins find themselves at the front of the diffusion. As a result, the magnetization vectors begin to reconverge. At some later time, all the spin vectors are aligned again in the same direction. This realignment creates a xe2x80x9cspin echoxe2x80x9d which can be recorded as an induced voltage in the receiver coil. As the time between the excitation pulse and the realignment pulse is increased, the spin echo amplitude decays. Neglecting microscopic molecular diffusion, the characteristic decay time is known as the spinxe2x80x94spin or transverse relaxation time and is denoted as T2. The amplitude of the spin echoes can be used to determine spin density, T1 and T2.
Oil-based muds are becoming increasingly prevalent in borehole drilling techniques. Current methods of determining dip formation, such as electrical resistivity sensors, do not operate well in the presence of these oil-based muds. NMR techniques, however, can work in an environment containing oil-based muds. In normal NMR procedures, the logging process is slower than more conventional methods. Power consumption is excessive, often more than 200 W. However NMR remains useful because it gives information on petrophysical parameters that otherwise are unobtainable.
A smaller device could use less material and less energy than current commercial devices. The invention described herein concerns itself with BVI and CBW measurements only. Due to the nature of these measurements, which can be performed closer to the device, the sensor itself can be is smaller. Reducing size cuts material and energy costs, and simultaneously improves the sensitivity and resolution of the machine. With the increased sensitivity, the logging speeds can also increase, thereby reducing costs further.
Reduced size also allows the device to be placed on a sensor assembly which can be placed to the side of the tool. The invention further introduces a multiple receiver coil assembly which creates a high-resolution log.
The present invention is a logging tool for nuclear magnetic resonance (NMR) logging of an earth formation. One or more sensor assemblies are coupled to the housing of the logging tool by an arm, enabling close contact with the borehole wall. The static magnetic field is produced by a horseshoe or U-shaped magnet. Separate transmitter and receiver coils are used for pulsing the formation with a radio frequency (RF) field and receiving spin-echo signals. Short CPMG sequences or modified CPMG sequences having a refocusing angle of less than 180xc2x0 may be used for the RF pulsing. This makes it possible to obtain estimates of CBW and BVI. High logging speeds are possible because of the short pulse sequences.
To further increase the logging speed, the axial extent of the transmitter coils and the permanent magnet is greater than that of the receiver coils. This ensures that the received signals correspond to substantially equal polarization of the nuclear spins even at high logging speeds.
When a plurality of sensor assemblies are used, azimuthal information about the parameters of interest is obtained. The arms are independently extendable and may be powered by a spring, hydraulic or electrical power. Differences of dip between the CBW and BVI are indicative of cross-bedding in the formation.
In a preferred embodiment, the U-shaped magnet includes two bar magnets connected at one end by a magnetically permeable core. Resolution of the device may be changed by altering the gap in the U-shaped magnet. Optionally, a coil around the core may be used as a field shifting electromagnet that makes possible the use of phase-alternated pulse sequences with little decrease in logging speed. When two or more receiver coils are used in a single sensor assembly, improved resolution is obtained using the phase of the received signals.